In a vehicle equipped with an anti-lock brake control device, a reference value for determining whether or not a wheel is about to become locked during braking is conventionally provided from a vehicle speed which is estimated based on detected values from wheel speed detectors, as disclosed in Japanese Patent Application Laid-Open No. 184558/88.
The estimation of the vehicle speed using the detected values from the wheel speed detectors, as described above, is not very accurate. If the vehicle speed is, instead, estimated from a detected value from an acceleration-deceleration detector mounted on the vehicle body, it is possible to estimate the vehicle speed reliably and accurately.
The vehicle may travel not only on a flat road surface but also on an upward or downward slope. While traveling on an upward or downward slope, an effect corresponding to a gradient is exerted on the detected value in the acceleration-deceleration detector which will effect the accuracy of the detected value.